There are many instances where it is required to drill horizontal boreholes from an underground drilling rig into a substantially horizontal seam in a mining operation. Such examples include, but are not limited to, the taking of geo samples from underground seams, and gas drainage, e.g. the draining of methane from underground coal seams. These techniques are commonly referred to as “in-seam drilling,”.
In-seam drilling is a significant cost component of underground mining, and in particular coal mining, with a high cost of setting up an in-seam drilling rig and high risk associated with the use of a downhole drilling motor and survey tool system.
Present in-seam drilling rigs typically use conventional drill strings with jointed components which is very labour intensive with manual handling of drill pipe and water swivel connections typically required for every three meters drilled. The normal operational crew of existing in-seam drilling systems is typically three people per shift and there are significant risk and cost benefits to be gained by reducing the general underground population and simplifying the drilling rigs used in this situation.
The use of coiled tubing which comprises a relatively thin walled strip of sheet metal coiled and edge-welded into a continuous tube which is able to transmit a longitudinal thrust force while being flexible enough to be wound onto a drum or passed around a bend has been known in drilling operations for some time. Coiled tubing operations were originally developed for workovers (treatment, re-stimulation, and maintenance) of existing oil and gas wells. The continuous roll of tubing allowed for rapid insertion and retraction of downhole tools, and enabled these operations to be completed without the need for a conventional workover rig. Coiled tubing drilling (CTD) has been in use for some time, typically for the placement of substantially vertical, slim hole wells (typically gas wells), although CTD technology has more recently been used for deep directional and horizontal wells. It is however typically difficult to control the direction of the drilling head in a CTD rig and the present invention addresses this problem in a manner which allows for cost effective and accurate deployment of a CTD drilling rig in an underground mining situation.
As an alternative to coiled tubing, it is also known to use composite tubing, and these alternatives are generically described as semi-rigid tubing in this specification.